问题描述：Implement the following operations of a stack using queues.
- push(x) -- Push element x to the back of queue.
- pop() -- Removes the element from in front of queue.
- peek() -- Get the front element.
- empty() -- Return whether the queue is empty.
Notes:
- You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
- Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
- You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
Python代码：
class MyQueue(object):
def __init__(self):
"""
Initialize your data structure here.
"""
self.stack = []
def push(self, x):
"""
Push element x to the back of queue.
:type x: int
:rtype: void
"""
swap = []
while self.stack:
swap.append(self.stack.pop())
swap.append(x)
while swap:
self.stack.append(swap.pop())
def pop(self):
"""
Removes the element from in front of queue and returns that element.
:rtype: int
"""
if self.stack:
return self.stack.pop()
def peek(self):
"""
Get the front element.
:rtype: int
"""
if self.stack:
return self.stack[-1]
def empty(self):
"""
Returns whether the queue is empty.
:rtype: bool
"""
return len(self.stack) == 0
